Nocturnal cyclic intermittent hypoxia, such as experienced by patients with obstructive sleep apnea (OSA) is thought to alter vascular tone and function leading to peripheral vasoconstriction and consequent arterial hypertension. Studies in patients indicate that sleep apnea results in diminished reactivity to endogenous vasodilators, and altered sensitivity to some endogenous vasoconstrictors. Furthermore, OSA patients demonstrate an augmented pressor response when exposed to hypoxia and fail to decrease forearm vascular resistance (FVR) when exposed acutely to progressive isocapnic hypoxia as do non-apneic volunteers. Our own preliminary data, collected in normal volunteers and OSA patients, suggests, however, that intermittent hypoxic exposure may lead to vasodilation rather than vasoconstriction. Normal volunteers exposed to intermittent hypoxia for 14 nights have no change in vascular resistance despite increased sympathetic activity and also fail to vasodilate when acutely exposed to isocapnic hypoxia after the repetitive exposure. Based on these and other observations we propose three hypotheses. First, we hypothesize that hypoxic exposure causes either sustained vasodilator release (e.g., epinephrine, NO) that persists during normoxia or altered sympathetic transduction with diminished vasoconstriction (e.g., altered receptor density or transmitter release), or both. Second, we speculate that an acute re-exposure to hypoxia after a prior intermittent exposure results in an abrupt increase in sympathetic nervous system activity but little further increase in vasodilator release, thus resulting in impaired vasodilation. Finally, we hypothesize that maximum vasodilator release declines over time with continued hypoxic exposure, resulting in a gradual increase in arterial pressure. To test these hypotheses we plan a series of investigations in normal volunteers before and after an exposure to cyclic nocturnal hypoxia for 28 nights, and in OSA patients before and after 30 days of monitored therapy with nasal CPAP. These studies will use selective intra-arterial infusions of specific pharmacological agonists and antagonists to assess the roles of endogenous vasodilators and vasoconstrictors in altering vascular tone following an exposure to hypoxia. We anticipate that these studies will significantly enhance our understanding of how intermittent hypoxia and obstructive sleep apnea influence vascular tone and function. [unreadable] [unreadable] [unreadable]